It's sad that I have to write this post.
Critique is feedback intended to educate. It may be pointing out flaws, highlighting errors, offering suggestions on approach, or even offering praise.
"This sucks" is not a critique. It's simply an opinion. Just like "I hate this" or "This is awful" or anything else. And to get smug about how "blunt" you are and how you're just being honest is bullshit. If you like being a dick, fine, you like being a dick. But admit you're a dick.
If you want to critique someone, critique them. If they get offended, they're operating from their ego and are sensitive, or unwilling to accept it, or ignorant. If they get shitty, they're the asshole, not you.
If, however, you intend to simply state how much something sucks, don't be a coward and hide behind "critique" -- at the very least, stand tall and admit you're simply being judgmental and dislike whatever you're discussing.
Hiding behind the facade of education when you decide to insult someone is bullying, plain and simple. You're trying to get your pot-shots in while remaining innocent and blameless. After all, you were just trying to help. No you weren't. You were being a dick, and now you're a liar.
If you've never made anything and you do this to people: Go fuck yourself.
If you make stuff and you do this to people: you should know better. Go fuck yourself twice.
If you want someone to get better: critique them.
You can have all the opinions in the world. You can hate stuff all you want. I don't care. What I DO care about is when you lie about your intentions, or try to have your cake and eat it too. Like I said, be as blunt as you want. Just admit you're being a dick when you do it.
(Also, when you receive a critique, "Not good enough for professional work" is valid and fair. If you want to get better, you'll try to get over hurt feelings and listen. If, however, you want to stay where you are while feeling bad, take it personally.)